The neutron crystal structure of 9-methyladenine at 126 K

At 126 K, 9-methyladenine (C6H7N5) is monoclinic, space group P21/c, with a=7.506(1), b=12.285(2), c=8.483(1) A, β=122.83(1)0, and four molecules per unit cell. The structure has been refined using 2983 reflections with a measured neutron wavelength of 0.9085 A, and 373 reflections at a wavelength of 1.0474 A. The latter consist mainly of 2h,2k,2l reflections which were remeasured when it was found that the shorter wavelength was contaminated with 2lambda neutrons produced by double Bragg diffraction at the Be monochromating crystal. The final R(F2) value was 0.049. Bond lengths and angles have e.s.d.'s of 0.001 A, 0.080 for non-hydrogen atoms, otherwise 0.003 A, 0.20. Values are in general agreement with those of the previously reported room-temperature X-ray refinement. The hydrogen bonding of the amino group appears to cause displacement of the H atoms from the molecular plane. The largest-thermal motion in the structure involves the methyl H atoms which are librating about the N(9)-CH3 bond with r.m.s. amplitudes 0.28, 0.29, 0.30 A. There is a short intermolecular distance of 2.52 A between a methyl H atom and N(3). (Auth.)